Services

The Tour of Heroes is evolving and we anticipate adding more components in the near future.

Multiple components will need access to hero data and we don't want to copy and paste the same code over and over. Instead, we'll create a single reusable data service and learn to inject it in the components that need it.

Refactoring data access to a separate service keeps the component lean and focused on supporting the view. It also makes it easier to unit test the component with a mock service.

Because data services are invariably asynchronous, we'll finish the chapter with a promise-based version of the data service.

Run the live example for part 4

Where We Left Off

Before we continue with our Tour of Heroes, let’s verify we have the following structure. If not, we’ll need to go back and follow the previous chapters.

Keep the app transpiling and running

Open a terminal/console window. Start the TypeScript compiler, watch for changes, and start our server by entering the command:

The application runs and updates automatically as we continue to build the Tour of Heroes.

Creating a Hero Service

Our stakeholders have shared their larger vision for our app. They tell us they want to show the heroes in various ways on different pages. We already can select a hero from a list. Soon we'll add a dashboard with the top performing heroes and create a separate view for editing hero details. All three views need hero data.

At the moment the AppComponent defines mock heroes for display. We have at least two objections. First, defining heroes is not the component's job. Second, we can't easily share that list of heroes with other components and views.

We can refactor this hero data acquisition business to a single service that provides heroes and share that service with all components that need heroes.

Create the HeroService

Create a file in the app folder called hero.service.ts.

We name the class HeroService and export it for others to import.

Injectable Services

Notice that we imported the Angular Injectable function and applied that function as an @Injectable() decorator.

TypeScript sees the @Injectable() decorator and emits metadata about our service, metadata that Angular may need to inject other dependencies into this service.

The HeroService doesn't have any dependencies at the moment. Add the decorator anyway. It is a "best practice" to apply the @Injectable() decorator ​from the start​ both for consistency and for future-proofing.

Getting Heroes

Add a getHeroes method stub.

We're holding back on the implementation for a moment to make an important point.

The consumer of our service doesn't know how the service gets the data. Our HeroService could get Hero data from anywhere. It could get the data from a web service or local storage or from a mock data source.

That's the beauty of removing data access from the component. We can change our minds about the implementation as often as we like, for whatever reason, without touching any of the components that need heroes.

Mock Heroes

We already have mock Hero data sitting in the AppComponent. It doesn't belong there. It doesn't belong here either. We'll move the mock data to its own file.

Cut the the HEROES array from app.component.ts and paste it to a new file in the app folder named mock-heroes.ts. We copy the import {Hero} ... statement as well because the heroes array uses the Hero interface.

We export the HEROES constant so we can import it elsewhere — such as our HeroService.

Meanwhile, back in app.component.ts where we cut away the HEROES array, we leave behind an uninitialized heroes property:

Return Mocked Heroes

Back in the HeroService we import the mock HEROES and return it from the getHeroes method. Our HeroService looks like this:

Use the Hero Service

We're ready to use the HeroService in other components starting with our AppComponent.

We begin, as usual, by importing the thing we want to use, the HeroService.

Importing the service allows us to reference it in our code. How should the AppComponent acquire a runtime concrete HeroService instance?

Do we new the HeroService? No way!

We could create a new instance of the HeroService with "new" like this:

That's a bad idea for several reasons including

• Our component has to know how to create a HeroService. If we ever change the HeroService constructor, we'll have to find every place we create the service and fix it. Running around patching code is error prone and adds to the test burden.

• We create a new service each time we use "new". What if the service should cache heroes and share that cache with others? We couldn't do that.

• We're locking the AppComponent into a specific implementation of the HeroService. It will be hard to switch implementations for different scenarios. Can we operate offline? Will we need different mocked versions under test? Not easy.

What if ... what if ... Hey, we've got work to do!

We get it. Really we do. But it is so ridiculously easy to avoid these problems that there is no excuse for doing it wrong.

Inject the HeroService

Two lines replace the one line of new:

1. we add a constructor.
2. we add to the component's providers metadata

Here's the constructor:

The constructor itself does nothing. The parameter simultaneously defines a private _heroService property and identifies it as a HeroService injection site.

Now Angular will know to supply an instance of the HeroService when it creates a new AppComponent.

Angular has to get that instance from somewhere. That's the role of the Angular Dependency Injector. The Injector has a container of previously created services. Either it finds and returns a pre-existing HeroService from its container or it creates a new instance, adds it to the container, and returns it to Angular.

The injector does not know yet how to create a HeroService. If we ran our code now, Angular would fail with an error:

We have to teach the injector how to make a HeroService by registering a HeroService provider. Do that by adding the following providers array property to the bottom of the component metadata in the @Component call.

The providers array tells Angular to create a fresh instance of the HeroService when it creates a new AppComponent. The AppComponent can use that service to get heroes and so can every child component of its component tree.

getHeroes in the AppComponent

We've got the service in a _heroService private variable. Let's use it.

We pause to think. We can call the service and get the data in one line.

We don't really need a dedicated method to wrap one line. We write it anyway:

The ngOnInit Lifecycle Hook

AppComponent should fetch and display heroes without a fuss. Where do we call the getHeroes method? In a constructor? We do not!

Years of experience and bitter tears have taught us to keep complex logic out of the constructor, especially anything that might call a server as a data access method is sure to do.

The constructor is for simple initializations like wiring constructor parameters to properties. It's not for heavy lifting. We should be able to create a component in a test and not worry that it might do real work — like calling a server! — before we tell it to do so.

If not the constructor, something has to call getHeroes.

Angular will call it if we implement the Angular ngOnInit Lifecycle Hook. Angular offers a number of interfaces for tapping into critical moments in the component lifecycle: at creation, after each change, and at its eventual destruction.

Each interface has a single method. When the component implements that method, Angular calls it at the appropriate time.

Here's the essential outline for the OnInit interface:

We write an ngOnInit method with our initialization logic inside and leave it to Angular to call it at the right time. In our case, we initialize by calling getHeroes.

Our application should be running as expected, showing a list of heroes and a hero detail view when we click on a hero name.

We're getting closer. But something isn't quite right.

Async Services and Promises

Our HeroService returns a list of mock heroes immediately. Its getHeroes signature is synchronous

Ask for heroes and they are there in the returned result.

Someday we're going to get heroes from a remote server. We don’t call http yet, but we aspire to in later chapters.

When we do, we'll have to wait for the server to respond and we won't be able to block the UI while we wait, even if we want to (which we shouldn't) because the browser won't block.

We'll have to use some kind of asynchronous technique and that will change the signature of our getHeroes method.

We'll use promises.

The Hero Service makes a promise

A promise is ... well it's a promise to call us back later when the results are ready. We ask an asynchronous service to do some work and give it a callback function. It does that work (somewhere) and eventually it calls our function with the results of the work or an error.

Update the HeroService with this promise-returning getHeroes method:

We're still mocking the data. We're simulating the behavior of an ultra-fast, zero-latency server, by returning an immediately resolved promise with our mock heroes as the result.

Act on the Promise

Returning to the AppComponent and its getHeroes method, we see that it still looks like this:

As a result of our change to HeroService, we're now setting this.heroes to a promise rather than an array of heroes.

We have to change our implementation to act on the promise when it resolves. When the promise resolves successfully, then we will have heroes to display.

We pass our callback function as an argument to the promise's then method:

Our callback sets the component's heroes property to the array of heroes returned by the service. That's all there is to it!

Our app should still be running, still showing a list of heroes, and still responding to a name selection with a detail view.

Review the App Structure

Let’s verify that we have the following structure after all of our good refactoring in this chapter:

Here are the code files we discussed in this chapter.

The Road We’ve Travelled

Let’s take stock of what we’ve built.

• We created a service class that can be shared by many components
• We used the ngOnInit Lifecycle Hook to get our heroes when our AppComponent activates
• We defined our HeroService as a provider for our AppComponent
• We created mock hero data and imported them into our service
• We designed our service to return a promise and our component to get our data from the promise

Run the live example for part 4

The Road Ahead

Our Tour of Heroes has become more reusable using shared components and services. We want to create a dashboard, add menu links that route between the views, and format data in a template. As our app evolves, we’ll learn how to design it to make it easier to grow and maintain. We’ll learn more about these tasks in the coming chapters.

Appendix: Take it slow

We can simulate a slow connection.

Add the following getHeroesSlowly method to the HeroService

Like getHeroes, it also returns a promise. But this promise waits 2 seconds before resolving the promise with mock heroes.

Back in the AppComponent, swap _heroService.getHeroesSlowly for _heroService.getHeroes and see how the app behaves.

Appendix: Shadowing the parent's service

We stated earlier that if we injected the parent AppComponent HeroService into the HeroDetailComponent, we must not add a providers array to the HeroDetailComponent metadata.

Why? Because that tells Angular to create a new instance of the HeroService at the HeroDetailComponent level. The HeroDetailComponent doesn't want its own service instance; it wants its parent's service instance. Adding the providers array creates a new service instance that shadows the parent instance.

Think carefully about where and when to register a provider. Understand the scope of that registration. Be careful not to create a new service instance at the wrong level.